The present invention relates generally to electrical conductors and switches and more particularly to a quantum wire logic gate.
Many kinds of electrical wires and switches are in widespread use in applications ranging from industrial motor controls to ordinary house wiring to high-speed solid state logic in microcomputers. A wire that carries current for a given electrical device must be physically large enough to carry the quantity of current required by the device without overheating. To minimize cost and physical bulk, wires and switches should be no larger than needed. The constraint of making wires and switches big enough to perform their functions and as small as possible to minimize size and cost is particularly critical in microprocessor fabrication because a very large number of wires and switches must fit into a very small space.
Innovations in semiconductor design and semiconductor manufacture have led to methods of making very tiny transistor switches that require extremely small amounts of electrical current. This has made it possible to shrink the size of the conductors that carry the current to and from the transistors to such an extent that millions of transistors and conductors can now fit into a semiconductor chip one or two centimeters square.
An electric current can be defined as a flow of charge carriers through a conductor. Countless millions of charge carriers can fit on a cross-sectional slice of a typical conductor such as house wiring, and even at the small dimensions found in today's integrated circuits many charge carriers can fit across one conductor.
Every physical object, including each of these charge carriers, has a wave nature associated with it. This means that in some respects the object behaves as if it were a particle and in other respects the object behaves as if it were a wave. The particle behavior is dominant in any object where the wavelength associated with the object, called the de Broglie wavelength, is much smaller than the other characteristic dimensions of the physical system in which the object is present. For all practical purposes, this means that an object large enough to be seen with the human eye will behave like a particle rather than like a wave. Newton's laws of motion describe the particle behavior of these objects.
In objects operating in systems that have characteristic dimensions approaching the size of the de Broglie wavelength of the object, the particle behavior is not dominant and the wave behavior becomes important. Newton's laws do not describe the wave behavior of objects; instead, one must have recourse to the principles of quantum mechanics.
Quantum mechanics is the study of systems that approach the size of the de Broglie wavelength where the wave nature of particles becomes dominant. The de Broglie wavelength associated with a charge carrier in a semiconductor such as gallium arsenide is of the order of ten nanometers (one nanometer is 10.sup.-9 meters). If the conductor in which such a charge carrier is moving has a cross-section only slightly larger than the de Broglie wavelength of the particle, the wave behavior of the charge carrier will become dominant and will correctly predict what will happen to the charge carrier as it moves through the conductor. One aspect of the wave behavior of objects is the occurrence of quantum interference effects.
Quantum interference effects are similar to interference effects observed in optical systems, such as interferometers and reflection gratings, but occur only for much smaller physical systems. The developing capability of fabricating smaller and smaller conductors has allowed researchers to begin to investigate electrical devices whose operational characteristics are dependent upon the quantum interference effects of charge carriers.
Much recent activity has centered on structures called "quantum wires". These structures are long thin channels which confine charge carriers within a region of maximum transverse dimension comparable to the de Broglie wavelength of those charge carriers. These quantum wires can act as fermionic waveguides, in direct analogy to electromagnetic waveguides. In order for the propagating modes of the quantum wire to maintain their quantum wave coherence along the length of the structure, it is necessary that the carriers enjoy "ballistic transport". This means the carriers must not undergo decohering inelastic scattering events as they propagate through the structure. Due to improvements in achievable materials characteristics, simple quantum wire structures have been fabricated and tested, as reported, for example, by Ismail, Bagwell, Orlando, Antoniadis and Smith in the Proceedings of the IEEE, volume 79, page 1106 (1991).
A means of controlling the conductance properties of the channel has been implemented by making a channel with a variable length stub of quantum wire attached in the transverse dimension. The conductance of the channel is affected by reflected standing waves which are established in the stub. These standing waves interfere with the channel wave function. The interference effects are dependent upon the de Broglie wavelength of the carriers and the length of the stub. If the channel and stub are fabricated as, for example, gallium arsenide (GaAs) channels in an aluminum gallium arsenide (AlGaAs) substrate, the length of the stub can be controlled by a bias voltage which varies a depletion region that determines the stub termination point. This simple structure has been called an "electron stub tuner" or "quantum stub tuner" and was independently proposed by Sols, Macucci, Ravaioli Hess, Applied Physics Letters, vol. 54 page 350 (1989) and vol. 66 page 3892 (1989), and by Fowler in U.S. Pat. No. 4,550,330, issued Oct. 29, 1985.
The conductance of the channel in an electron stub tuner is expected to be a periodic function of the stub length, and this is borne out by data descriptive of actual electron stub tuners (see Aihara, Yamamoto and Mizutani, International Electronic Devices Meeting, 1992).
The electron stub tuner and other possible quantum wire structures can be theoretically analyzed using a simple model wherein the wire channels are assumed to have negligible transverse spatial extent. This allows analysis of the structure by solving the one-dimensional Schrodinger's equation on a thin-wire network wherein the wavefunctions and boundary conditions are well-defined, as described in Ruedenberg and Scherr, Journal of Chemical Physics, vol. 21, page 1565 (1953).
If quantum wire structures of more complicated topologies than the electron stub tuner could be developed, it would be possible to create an entirely new class of electronic devices that would offer significant advantages over conventional devices in terms of smaller size, higher speed, and lower power consumption. Accordingly, it will be apparent that there is a need for quantum wire devices having more capabilities than the electron stub tuner.